Silicon is often used as a semiconductor material for many purposes. The surface of silicon is very chemically active; and easily combines with many materials, including oxygen. The xe2x80x9coxidexe2x80x9d formed on the surface can impede the characteristics of the silicon.
Semiconductors having band gaps between 0.9 and 1.7 eV are often used in photoelectrochemical solar energy conversion devices. Silicon has a band gap of 1.12 eV, and is an especially attractive material for this application.
A typical way of forming a silicon semiconductor, e.g., a photodiode, is to use wet chemical etching with HF(eq), resulting in a hydrogren-terminated Si surface. This surface is electrically suitable when initially formed. However, this surface readily oxidizes in air or in water-containing ambients. An oxide on the surface introduces electrical defect states. This also forms an insulating, passivating overlayer. That overlayer can impede the semiconductor characteristics. For example, this can impede photocurrent flow through an electrochemical cell.
It is often important to stabilize Si electrodes in aqueous media. Previous art has suggested coating the surface of Si electrodes with islands or films of metal. This process creates buried Si/metal junctions on protected regions of the electrode. This can be undesirable.
The present specification describes silicon surfaces derivatized with covalently-attached alkyl chains used to protect the surface. A preferred mode uses a halogenation/alkylation procedure in which the surface is treated with a halogen, e.g., chlorine, and then an alkyl-containing material. The silicon surface is preferably in contact with an aqueous Fe(CN)63xe2x88x92/4xe2x88x92. These surfaces are treated with the alkyl groups, or xe2x80x9cderivatized.xe2x80x9d This forms covalent Sixe2x80x94C linkages and these covalent Sixe2x80x94C linkages protect the surface against oxidation.
A preferred mode uses CH3 as the alkyl group.